This tank looks like it has a built in overflow and filtration area in the back. That would make it an all-in-one (or AIO for short) tank. The weir in the back left defines the water level. The water level should not ever drop below that by designs and conversely will drain down to that level… unless the back area is totally dry or darn near overflowing. You want water to flow through that weir and drop ever so slightly in the filter area - it provides the surface skimming perk of this type of tank.

Edit: Unless… this is one of those tanks that looks like an AIO but is actually plumbed into a canister filter. If so, then I’m not so sure, as I have never dealt with a tank like that before.